Volvation

Caecosphaeroma burgundum: two of three pillbugs have curled themselves into "pills"

Volvation (from Latin volvere "roll", and the suffix -(a)tion; sometimes called enrollment or conglobation), is a defensive behavior in certain animals, in which the animal rolls its own body into a ball, presenting only the hardest parts of its integument (the animal's "armor"), or its spines to predators.

Among armadillos, only species in the genus Tolypeutes (South American three-banded armadillos) are able to roll into a defensive ball; the nine-banded armadillo and other species have too many plates.^[1]

Volvation is used by earthworms during periods of extreme heat or drought.

Among pill millipeds, volvation is both a protection against external threats and against dehydration.^[2]

Pillbugs curl themselves into "pills" not only for defense, but also to conserve moisture while resting or sleeping, because they must keep their pseudotrachaea ("gills") wet. Volvation is particularly well evolved in subterranean isopods, but only Caecosphaeroma burgundum is able to roll up into a hermetic sphere without any outward projections, and thus "approaches perfection in volvation".^[3]

In vertebrates, an animal's decision to volvate is mediated by the periaqueductal gray region.^[4]

Gallery[]

• Examples of species that practice volvation:
• 

  The European hedgehog (Erinaceus europaeus).

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  The Southern three-banded armadillo (Tolypeutes matacus).

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  An Indian pangolin (Manis crassicaudata) defending itself against Asiatic lions (Panthera leo persica).

• 

  Two lesser hedgehog tenrecs (Echinops telfairi).

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  Short-beaked echidna (Tachyglossus aculeatus) curling into a ball.

• 

  The ball python (Python regius) curls into a ball when stressed or frightened.

• 

  The armadillo girdled lizard (Ouroborus cataphractus).

• 

  The common pillbug (Armadillidium vulgare) is named after the pill it resembles when curled up.

• 

  Phacops rana, a trilobite from the Devonian period.

• 

  Tachypodoiulus niger, a millipede, with legs on the inside and head in the center.

• 

  Stages of a pill millipede (Glomeris marginata) unrolling.

References[]